1. Field of the Invention
The invention generally relates to reactive ion etching systems for photomask fabrication upon a reticle and, more particularly, to a reticle adapter that facilitates dry etching a photomask using a conventional reactive ion etch chamber.
2. Background of the Invention
Semiconductor lithography is accomplished using a reticle comprising a relatively thick substrate typically fabricated of quartz, having a photomask fabricated of chrome patterned upon the surface of the substrate. A reticle is generally 5 inches by 5 inches, 6 inches by 6 inches or 9 inches by 9 inches. To fabricate the photomask pattern, a uniform submicron (e.g., 0.1 to 1.0 micron) thick layer of chrome has a photoresist layer applied thereto. The photoresist is then patterned using conventional laser or electron beam patterning equipment. The chrome layer is then etched using a wet etch process to remove material not protected by the photoresist. The isotropic characteristic of a wet etch causes an undercut phenomenon to occur below the photoresist material such that the chrome lines that are patterned upon the reticle are not uniformly spaced nor do they have vertical side walls. Such undercut on photomask patterns that will be used to produce sub 0.25 xcexcm devices can result in erroneous lithography.
As semiconductor devices shrink in size, the on-mask chrome structures must also experience a corresponding reduction in size. The use of optical proximity correction (OPC) structures that are generally needed to minimize lines for shortening and corner rounding have necessitated significantly smaller features to be etched into the chrome. With the need for OPC structures and the small line widths of the chrome features, the use of wet etching to form photomasks has become undesirable. Additionally, the environmental concerns for disposal of the by-products of a wet etch process have also led to wet etch photomask processing""s undesirability.
Plasma etch processing (also known as dry etch processing) provides an alternative to wet etching and can provide superior photomask fabrication results. Dry etch of photomasks for lithography reticles has been shown to improve resolution, critical dimensions and critical dimension linearity as well as take advantage of the anisotropic etch characteristics such that very little or no undercutting is experienced. Although reactive ion etching to form photomasks has been shown to have superior results compared to wet etch, the industry has been slow to design new and costly dry etch chambers and equipment to facilitate photomask generation. Conversion of traditional wafer etching equipment that is intended to handle 8 inch (200 mm) circular wafers into a system for handling 5, 6 or 9 inch square reticles typically requires replacement of the wafer support pedestal. This is generally costly and complicated.
Therefore, there is a need in the art for an adapter that can be used to convert a conventional reactive ion etch chamber into a chamber capable of dry etching a photomask without substantial cost or complicated equipment.
The disadvantages associated with the prior art are overcome by the invention of a reticle adapter that is capable of supporting a square reticle in a conventional reactive ion etch (RIE) chamber that is designed for etching circular silicon wafers. In short, the adapter converts the circular pedestal into a square photomask reticle support. The adapter is fabricated from a non-reactive material such as polyimide, ceramic or quartz.
More specifically, the adapter has a base portion that is milled to conformally cover the entire upper portion of a wafer pedestal such that plasma cannot contact the pedestal. The adapter contains a base portion that has a circular plan form and an inner surface that follows the contours of the top of the pedestal. The reticle rests in the upper portion of the adapter, spaced apart from the pedestal upper surface. As such, the reticle backside will not contact the pedestal surface and become contaminated or scratched. The reticle backside is only contacted by the adapter at its four corners or its edges such that the central portion of the reticle through which light passes during lithography is not contacted by the adapter nor any part of the reactive ion etch chamber.
The adapter is designed to fit over the electrostatic chuck of a reactive ion etch chamber such as the decoupled plasma source (DPS) metal etch chamber fabricated by Applied Materials, Inc. of Santa Clara, Calif. Once supported in the chamber a plasma is generated above the reticle such that the photomask is etched in approximately 2 minutes to remove the excess chrome that is not protected by photoresist material and leave a photolithography pattern behind.
The reticle adapter can be also outfitted with mechanical restraining elements to hold the reticle within the adapter such that once the reticle is loaded into the adapter it never needs to be contacted by either robotics or human hands. As such, the reticle can be easily carried from place to place and chamber to chamber as processing is accomplished. One embodiment of a mechanical retainer consists of four either spring loaded or threaded screws that are screwed into the sides of the adapter and can be tightened to apply force to the reticle such that the reticle will not fall out of the adapter once the screws are tightened. Consequently, the invention is not only an adapter, but a reticle carrier as well.